I'd recommend It's Not the Stork. Your library may have it. The information is presented in a factual, clear, non-judgmental way and is age-appropriate for an 8 y.o.
In my house, the problem of how to talk about it was solved by one conversation in the car. My kids had watched a lot of nature documentaries, and my then 7 y.o. asked his big sister, a wise old woman of 10, why baboons have colorful butts. "To attract a mate," she said. He knew about mating because he had witnessed brine shrimp mating in real life. "Mating is for making babies, right?" "Yep." "I suppose people must do that, too." (By now I'm trying to listen intently while not wrecking the car.) " I guess. Mom, can we play Pokemon when we get home?"